1. Field of the Invention
The present invention relates to a method and apparatus for cutting continuous ribbons of flexible plastic material suitable for use as interlayer material in laminated automobile windshields into a series of discrete sheets, each having a preselected trapezoidal shape. The present invention has particular use in the fabrication of interlayers having a band of dye disposed along one longitudinal edge margin of the interlayer destined to be located along the upper edge of the laminated windshield when the latter is installed in an automobile.
Recent designs of automobile windshields require the lamination of bent glass sheets that are substantially trapezoidal in outline. The plastic interlayers for such sheets are usually supplied in the form of a continuous ribbon mounted on a roll. In the past, such ribbons have been cut into rectangular shapes by rotating the ribbon about a reel to form several layers or plies and cutting the layers transversely to form a series of rectangular sheets whose length approximated the outer perimeter of the reel. A considerable amount of plastic has been lost through trimming two triangular-shaped portions from the opposite ends of the rectangular sheets to convert from rectangular to trapezoidal shapes. Furthermore, the majority of automobile windshields are provided with a dyed upper portion. The purpose of this tinted or dyed portion is to serve as a means for avoiding glare and to reduce the intensity of overhead radiation entering into the vehicle and annoying the driver and his passengers.
It is usually desirable that the colored strip portion extend along the top portion of the windshield. The lower edge of the colored or tinted strip should extend generally horizontally when the windshield is installed across the front of the automobile in an inclined position. Since the windshields of modern cars have a continuous curve from one side to the other side, it is necessary to stretch the interlayer in such a manner that the edge portion that is colored, tinted or dyed is stretched to a lesser extent than the undyed portion forming the lower portion of the windshield when the latter is installed.
In the past, dyed plastic has been stretched differentially either on adjustable shaping frames or shaping cones. The shaping frames stretch one sheet at a time. The sheet must be mounted on the frame before stretching and removed after stretching, a time-consuming operation. Shaping cones permit the winding of several layers, which are stretched simultaneously, or the continuous movement of a continuous ribbon thereacross. Both of these latter techniques are faster than the adjustable frame technique for stretching plastic differentially, but do not make it possible to cut individual interlayer sheets of the proper size (particularly length) and proper trapezoidal shape for more than a single production pattern while the continuous ribbon is wound on the stretching cone in its differentially stretched condition.
2. Description of the Prior Art
U.S. Pat. No. 2,933,759 to Startzell discloses a plastic stretching apparatus that resembles an umbrella for stretching partially dyed plastic differentially. The ribs of the umbrella extend peripherally around a center shaft so that the ribs of the umbrella are capable of moving in unison from a generally cylindrical shape to a conical shape. The Startzell patent discloses more than one embodiment. In one of these embodiments, links of different lengths are provided on the opposite axial ends of the umbrella-like structure to expand the opposite axial ends radially or circumferentially in different planes transverse to the center shaft simultaneously and at different rates. Thus, the Startzell stretching apparatus is capable of stretching sheets differentially depending upon the relative lengths of the links that pivotally connect sleeves mounted on the center shaft to each end of the ribs of the umbrella. The Startzell stretching umbrella is thus able to convert a continuous ribbon of partly dyed or tinted plastic from a ribbon having a rectangular shape into a ribbon having a curved border between the dyed and undyed portions of the plastic ribbon. The structure of the stretching umbrella has to be rebuilt or a different umbrella has to be provided for each different pattern of windshields to be produced.
U.S. Pat. No. 3,467,332 to Bachman discloses a mandrel suitable for cutting an elongated ribbon of clear plastic into trapezoidal lengths that interfit with one another so that each two lengths of plastic that are cut from each circumferential ply of the plastic sheeting interfits. Clear plastic sheeting does not have to be distorted in the manner needed for plastic having a dyed portion. While the plastic cutting apparatus of the Bachman patent discloses means for adjusting the length of the periphery of the plastic layers or plies and provides means for orienting a pair of guides for cutting devices mounted on the cutting reel, this apparatus is incapable of use with partially dyed plastic except in the wasteful manner provided by prior art apparatus which required the plastic to be cut to rectangular shapes and then the triangles removed from the opposite longitudinal ends in order to provide the trapezoidal shapes desired.
U.S. Pat. No. 3,696,186 to Stark et al discloses an adjustable stretching cone having an adjustment mechanism operated by a crank in a standard manner to change the diameter of the cone along its narrow end. A plastic ribbon wound about the cone is differentially stretched an amount that depends upon the relative circumference of the smaller end of the cone compared to its larger fixed end. After the continuous ribbon is differentially stretched, it is cooled and cut to finite lengths. Each finite length has its lateral edges stretched to curved configurations of different arc lengths. According to this apparatus, each length is cut individually from the continuous ribbon. The cutting operation may thus limit the rate at which the individual sheets or lengths of plastic are fabricated from the continuous ribbon. On the other hand, the distorting or differential stretching of the continuous ribbon is performed while the conical stretching frame is rotated and this provides a limited time to differentially stretch the plastic and permit the plastic to be cooled throughout its thickness to a temperature at which it becomes permanently set in its deformed shape resulting from the differential stretching of its opposite side edges.
Thus, prior to the present invention, the glass laminating art lacked a method and apparatus that made it possible to cut a plurality of individual sheets of different sizes and trapezoidal shapes as needed from a single continuous ribbon of partially dyed plastic.